The goal of this program of research is to contribute to Science's understanding of the Hydrophobic Effect. Water, the 'solvent of life' has a profound influence on the structure and assembly of proteins and other biomolecules, yet there are still many unknowns regarding the modus operandi of the Hydrophobic Effect. For example, our understanding of the Hofmeister Effect - why some salts (kosmotropes) decrease the solubility of organic solutes whilst others (chaotropes) increase solubility - is poorly understood; even though the phenomenon was first described by Hofmeister over 120 years ago. In studying the formation of a host-guest complex driven by the hydrophobic effect, we have observed that the strength of complexation varies as a function of co-solute salts, in a manner paralleling the ability of salts to induce either precipitation or solubilization of proteins. Thus, kosmotropic sats cause an apparent decrease in the solubility of the host-guest pair and lead to an enhancement of the binding affinity, whilst chaotropes have the opposite effect. Using a combination of Isothermal Titration Calorimetry (ITC) and Nuclear Magnetic Resonance (NMR) spectroscopy, we have traced the ability of chaotropes to weaken binding to the fact that anions have a surprisingly strong affinity for hydrophobic concavity. In other words, the reduced affinity between host and guest occurs because chaotropic anions compete with the hydrophobic guest for binding to the host. This is the first observation of anions binding to hydrophobic concavity. Furthermore, ITC and NMR spectroscopy allows the accurate determination of the thermodynamics of host-guest and host-anion binding. As a result, the data we are gathering is allowing us to build the first molecular-scale models of the Hofmeister Effect. The major hypothesis behind this program of study is that anion binding to concavity is one of the major driving forces behind the observation that chaotropes break up protein quaternary and tertiary structure to form the molten-globule state. To build on this idea, this proposal describes experiments to probe the thermodynamics of 1:1 complexation of organic guests to a series of cavitand hosts. These studies will utilize a combination of ITC, NMR, spectroscopy, in silico work, and X-ray crystallography, to examine how co solutes salts influence these binding events. This data will be used to build the first thermodynamic models of the Hofmeister Effect at the molecular level, and has the potential to unify current models of the Hofmeister Effect based on bulk properties such as solubility, viscosity, and surface tension.